short circuit guide

7/28/2019 Short Circuit Guide

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MSC Guidelines for Review of Short

Circuit Analysis

Procedure Number: E2-19 Revision Date: 04/28/00

U.S. Coast Guard Marine Safety Center

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a. 46 CFR 111.51b. 46 CFR 111.52c. IEEE Standard 141-1976, Recommended Practice for Protection and

Coordination of Commercial Power Systems

d. Work Instruction E2-4, Coordination Studye. Work Instruction E2-7, Electrical One-Linef. Work Instruction E2-7, Ships Service Generator / Switchboardg. Matthews, John H. 1993.An Introduction to theDesign and Analysis of Building

Electrical Systems, New York: Chapman and Hall.

These guidelines were developed by the Marine Safety Center staff as an aid in thepreparation and review of vessel plans and submissions. They were developed tosupplement existing guidance. They are not intended to substitute or replace laws,regulations, or other official Coast Guard policy documents. The responsibility todemonstrate compliance with all applicable laws and regulations still rests with the plansubmitter. The Coast Guard and the U. S. Department of Transportation expresslydisclaim liability resulting from the use of this document.

If you have any questions or comments concerning this document, please contact theMarine Safety Center by e-mail or phone. Please refer to the Procedure Number:E2-19

E-mail: [email protected]: 202-366-6480.

An important safety aspect of electrical systems is the ability of components towithstand the mechanical and thermal forces associated with short circuits(faults). Protective devices such as fuses and breakers must not only have the

appropriate trip setting, but must be rated not to explode when subjected tocurrents many hundreds of times greater than their trip setting (as would beseen in a fault). Additionally, bus bars and panel boards must be rated so as notto rip apart from the intense mechanical forces associated with fault currents.This document outlines the basic procedure for ensuring that electrical systemcomponents are rated appropriately based upon the associated maximumasymmetrical fault current level.

References

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Information

Background


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Circuit Analysis



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Plan submittal and regulatory requirements are divided into two categories:systems below 1500 kW, and systems above 1500 kW in total aggregategenerating capacity. Systems above 1500 kW require calculation of short circuitcurrents using an established method. Systems below 1500 kW do not requirethis level of detail and employ assumptions.

Systems less than 1500 kW: Check the drawing notes for a minimum rating of all panelboards and

protective devices. This should be on the electrical one-line. Also check the

switchboard diagram for bus rating.

For an AC system, the max asymmetrical system short circuit current isconsidered to be 10 times the aggregate normal rated generator currents plusfour times the normal rated currents for all connected motors. All systemcomponents should be rated above this value (46 CFR 111.52-3(b) & (c))

For a DC system, the maximum short circuit current is considered to be 10times the aggregate normal rated generator currents plus six times the normalrated currents for all connected motors. All system components should berated above this value. (46 CFR 111.52-3(a))

For systems above 1500 KW, a separate submittal must address the short circuitanalysis (46 CFR 111.52-5). This submittal should contain a summary of themaximum fault currents under the following conditions:

3-phase bolted fault on the load side of each protective device fault on the distribution bus with all generators running in parallel supplying the

largest possible motor load

The summary should include enough data to verify short circuit calculations. Thisdata includes, but is not limited to: generator and motor ratings and impedances,transformer per-unit impedence, rating and reactance to resistance (X/R) ratio,cable type, length and impedance. Check the summary to ensure that the computedvalue of maximum asymmetrical fault current does not exceed the associatedprotective device rating. Reference the electrical one line and switchboard diagramsas applicable.

General Review

Guidance

Systems above

1500 kW


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Circuit Analysis



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AC fault currents are termed asymmetrical because the first cycle of the fault istypically much larger than subsequent cycles. It is this first cycle that generates themechanical force that system components must be able to withstand. For atheoretical background on this phenomenon, see reference (e).

Shipboard generator ratings determine the maximum fault current level. Themaximum current a generator will supply is determined by its impedance value. Ingeneral, the closer a fault is to a shipboard generator, the higher the value of faultcurrent.

Independently verify the following fault currents to see if they are within areasonable range:

Main distribution bus (switchboard). Electrical propulsion, as applicable. Steering system feeders. Fire fighting systems.

The above systems are considered vital and are the subject to the coordinationrequirements of 46 CFR 111.51-3 (see MSC ProcedureE2-4, Coordination Study)

During a bus fault, current is supplied from all the ships service generators andmotors connected to the bus. Current is supplied from motors due to their excessrotational inertia. All feeders that are not connected to motors or generators willcontain insignificant values of current because their impedance is high relative to thefault impedance. Therefore, only motor and generator feeders are consideredduring a bus fault.

To compute a main bus asymmetrical fault, first determine the individualimpedances from the motors and generators to the bus. These impedances are thenadded in parallel to determine the total impedance. The maximum symmetricalfault current is the system voltage divided by this value. An asymmetricalmultiplication factor is then applied; this factor can be found using IEEE STD 141-

1976, pages 205-206.

A fault in a non-motor/ generator feeder is computed as above, except theimpedance of the feeder in which the fault occurred is added in series with theimpedance computed for the bus fault.


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A fault in a motor/generator feeder is computed as follows: Add all of the motor/generator to bus impedances in parallel, with the

exception of the fault feeder impedance.

Take this value and add it in series with the fault feeder impedance. Divide the system voltage by this resultant aggregate impedance value. Multiply the divided voltage by the asymmetrical multiplication factor.

Typical impedance and X/R ratio values for rotating machines and cable can befound in IEEE STD 141-1976, pages 197 to 202.

The typical per-unit impedance of a transformer is around 5%.

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